1. Field of the Invention
The present invention relates to a handoff method of mobile communications systems such as CDMA Code Division Multiple Access communications system, mobile communication system, PCS Personal Communications Service, and satellite communications system, and more particularly to, a handoff method of mobile communications system in which whether a handoff is to be performed or not is decided after detecting pilot signal strengths of a mobile station and a base station which has been already connected, so that it is possible to control a handoff generation number, preventing unnecessary load.
2. Description of the Conventional Art
In general, as shown in FIG. 1, a mobile communication system for communicating while movement is composed of mobile station 10 in which communication service of a mobile communication subscriber is requested and communication service of a mobile communication system is provided, base station 20 for exchanging signals with the mobile station 10 at a wireless area, control station 30 for control the base station 20 to perform signal process between the base station 20 and a switching station 40, and the switching station 40 which is connected to the control station 30 and performs switching connection between mobile communication nets, thereby connecting a mobile communication net with another communication net.
The operation of the general mobile communication system, which is constructed as above, is as follows.
When a mobile communication subscriber having a mobile station is in a service area of the base station 20 and requests a mobile communication service, the switching station 40 detects a position of the mobile station 10 under the control of the control base 30 and performs mobile communication service according to the request of the mobile station 10.
FIG. 2 shows boundary areas of base stations of a general mobile communications system.
An area in which communication service from one base station is possible is called a cell. The cell is defined by pilot signal strength, which is output through a pilot channel from the base station. Therefore, in case that a mobile station is positioned at a cell boundary, it is possible to realize a base station of which service area a mobile station belongs to by detecting an input pilot signal strength of the mobile station which is transmitted to the base station.
When the mobile station 10 is located in a service area of a first base station 21 and removes in a service area of a second base station 22, the communicating mobile 10 has to perform handoff to keep on the communication. Wherein the handoff means an operation to automatically convert communication line to keep the mobile station""s communication when the mobile station removes from one cell to another cell.
FIG. 3 is a flow chart for explaining a handoff method of a conventional mobile communications system.
As shown in FIG. 3, the handoff method includes the steps of comparing a pilot signal strength, which is input from the mobile station to a neighboring base station, with a pilot detection threshold value (steps ST1-ST2), and transmitting handoff request message if the pilot signal strength of the neighboring base station is over than the pilot detection threshold value (step ST3-ST4).
Now the handoff method of the conventional mobile communications system will be described in more detail with reference to FIG. 1 and FIG. 2.
First, when the mobile station 10 is in the service area of the first base station 21 and served by the first base station 21, the pilot signal strength of the first base station 21 is detected and a signal detection message is transmitted from the mobile station 10 to the first base station 21.
The mobile station 10 approaches to a handoff area and, in step ST1, when the pilot signal strength of the second base station 22 among various pilot signal strengths is larger than the pilot detection threshold value, a signal detection message is transmitted to the first base station 21 for handoff to the second base station 22. The signal detection message includes information as to pilot off-set and the pilot signal strength for distinguishing the base stations 21 and 22.
In step ST2, the received signal detection message is transmitted to a base station controller by the first base station 21 and the pilot signal strength of the second base station 22 is compared with the pilot detection threshold value by the base station controller. In steps ST3 and ST4, a handoff to the second base station 22 is requested when the pilot signal strength of the second base station 22 is larger than the pilot detection threshold value, as a result of the comparison.
According to the handoff to be performed as above, it becomes possible to continuously communicate even when the mobile station 10 removes from the service area of the first base station 21 to that of the second base station 22.
However, the conventional communications system has disadvantages that the pilot signal strength of a base station which is serving the mobile station 10 is not considered so that unnecessary handoff may be caused, increasing handoff frequency, even in case that the pilot signal strength of the base station currently serving the mobile station 10 is larger than the mobile station requires for communication.
Further, as the handoff generation frequency increases, unnecessary overload is applied to the mobile communications system.
Therefore, the present invention is derived to resolve the disadvantages of the conventional techniques and it is an object of the present invention to provide a handoff method of a mobile communications system wherein, in case that a cell is defined by a sector, it is decided whether a handoff is to be performed or not after detecting pilot signal strengths of a mobile station and a base station which is already connected to the mobile station and then detecting pilot signal strengths of neighboring base stations, so that handoff generation frequency is efficiently controlled, preventing unnecessary overload of the system.
It is another object of the present invention to provide a handoff method of a mobile communications system wherein, in case that a cell is defined by a plurality of sectors, it is decided whether a handoff is to be performed or not after detecting a pilot signal strength of a sector which is in service and then detecting pilot signal strengths of neighboring sectors, so that handoff generation frequency is efficiently controlled, preventing unnecessary overload of the system.
In order to achieve the above objects of the present invention, according to an aspect of the present invention, a handoff method of a mobile communications system, in which a cell is defined by a sector, characterized in that a handoff request is made to a base station having a predetermined level of pilot signal strength by using a pilot signal strength of a base station which is in service, a predetermined threshold value and pilot signal strength of neighboring base stations.
In order to achieve the above objects of the present invention, according to another aspect of the present invention, a handoff method of a mobile communications system, in which a cell is defined by a sector, includes a first step of receiving a signal detection message from a mobile station and comparing a pilot signal strength of a base station which is in service with a threshold value for pilot checking of neighboring base stations, a second step of determining that the neighboring base stations are required to be checked their pilot signal strengths if the pilot signal strength of the base station which is in service is smaller than the threshold value for checking pilot of neighboring base stations, and comparing the pilot signal strength of the base station which is in service with pilot signal strengths of the neighboring base stations, and a third step of distinguishing a pilot signal strength of a predetermined level among the pilot signal strengths of the neighboring base stations if the pilot signal strength of the base station which is in service is smaller than the pilot signal strengths of the neighboring base stations, and transmitting a message for requesting a handoff to a neighboring base station which has the distinguished pilot signal strength.
In order to achieve the above objects of the present invention, according to another aspect of the present invention, a handoff method of a mobile communications system, in which a cell is defined by a plurality of sectors, includes a first step of detecting a sector corresponding to a predetermined level of pilot signal strength by using pilot signal strength of a sector which is in service, a predetermined threshold value and a pilot signal strength of other sectors, and determining whether the found sector belongs to a base station which is currently in service, and a second step of requesting a handoff between sectors in the base station which is currently in service if the found sector belongs to the base station in service, while requesting a handoff to another sector of neighboring base stations if the found sector does not belongs to the base station in service.